Calculation of Phase Diagrams of Polyetherimide/N, N-Dimethylacetamide/Nonsolvent Casting Systems and Prediction of Performances of Ultrafiltration Membranes

Calculation of phase diagrams of polyetherimide/N, N-dimethylacetamide/nonsolvent casting systems and prediction of performances of ultrafiltration membranes

Ming Tan, Gaohong He[*], Yan Dai, Rujie Wang, Junli Xiao

State Key Laboratory of Fine Chemicals, R&D Center of Membrane Science and Technology, School of Chemical Engineering, Dalian University of Technology, Dalian, 116024

Abstract: Polyetherimide (PEI)/N, N-dimethylacetamide (DMAc) ternary phase diagrams with nonsolvent (NS) of H₂O, polyethylene glycol (PEG400) and 1-butanol (BuOH) were calculated by using Hildebrand and Hansen solubility parameter and Flory-Huggins theory. The phase diagrams indicate that it should use Hansen solubility parameter and correct constant ¦Á of 1 to calculate interaction parameters of NS/DMAc/PEI casting system, and then the interaction parameter between BuOH solution and PEI was computed. Besides, PEI/DMAc/H₂O-BuOH quaternary phase diagram was simulated, which is consistent with the experimental cloud points. The phase diagrams show that when volume ratio of BuOH to H₂O is 9.327, 5.5% BuOH solution consisted of 5% BuOH and 0.5% H₂O leads to phase separation of 15-20wt.% PEI casting solution. Simultaneously, when volume ratio of BuOH to H₂O is 22.86, 6.5% BuOH solution including 6.2% BuOH and 0.3% H₂O brings on phase separation of the casting solution. Therefore, additive BuOH can enhance the phase separation of the PEI casting solution and hereby induce higher porosity of the membrane. Moreover, molecular size of BuOH is bigger than DMAc. Hence, compared with DMAc, diffusion of BuOH into the water coagulation bath causes larger pore size. However, when additive BuOH increases to a certain value, the greatly increased viscosity of the casting solution hinders the outlet of the solvent DMAc, additive and inlet of the nonsolvent water and thereby causes a thicker skin layer. As a result, it can be predicted that membrane PWF first increases then decreases, and RR decreases with the increases of BuOH concentration. The prepared membranes validated our predictions.

Key words: phase diagram; solubility parameter; interaction parameter; ultrafiltration; Flory-Huggins

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[*] Corresponding author Tel.: +86-411-84707892; Fax: +86-411-84707700; E-mail: hgaohong@dlut.edu.cn